1. Field of Invention
The present invention relates to the collection and temporary storage of mercury contaminants, more specifically the collection and temporary storage of used dental amalgam in a treatment facility.
2. Description of the Prior Art
Mercury is a unique metal in that it has an exceptionally high vapor pressure of 0.001201 torr at 20° C., which corresponds to a saturation concentration in air of 13.2 mg/m3. Table 1 shows the Hg concentrations in air increase rapidly with increasing temperature. At 20° C., the saturation concentration of Hg in air is 132 times the OSHA permissible exposure limit.
TABLE 1Vapor pressure and saturation concentrations of mercuryin air at selected temperatures.Temperature,Temperature,Vapor PressureHg Concentration,° C.° F.(torr)mg/meter30320.0001852.210500.0004905.920680.00120113.230860.00277729.5401040.00607962.6
The high vapor pressure of mercury can result in mercury vapor concentrations that exceed both Occupational Safety and Health Administration (OSHA) and The National Institute for Occupational Safety and Health (NIOSH) exposure limits if stored at room temperature. The current OSHA permissible exposure limit (PEL) for mercury vapor is 0.1 milligram per cubic meter of air (mg/m3) as eight-hour time weighted average (TWA). National Institute for Occupational Safety and Heath Recommended Exposure Limit (REL) for Hg vapor is 0.05 mg/meter3 for up to a 10-hour workday and 40-hour workweek. American Conference of Governmental Industrial Hygienists (ACGIH) has an Hg threshold limit value (TLV) of 0.025 mg/meter3 as a time-weighted average for an 8-hour workday and a 40-hour workweek (Occupational Safety and Health Guideline for Mercury Vapor, Dept. of Labor, http://www.osha.gov/SLTC/healthguidelines/mercuryvapor/recognition.html. 7 Mar. 2007).
TABLE 2Human exposure limits to elemental Hg vapor fromOSHA, NIOSH, and ACGIH.Type of LimitInorganic Hg CompoundsFederal OSHAa8-Hour TWA 0.1 mg/meter3 (100,000 ng/meter3)NIOSHb8-Hour TWA 0.05 mg/meter3 (50,000 ng/meter3)ACGIHc8-Hour TWA0.025 mg/meter3 (25,000 ng/meter3)
An amalgam is any mixture of mercury with another metal or an alloy, such as silver, tin, copper, and zinc (Webster's New College Dictionary. Houghton Mifflin Company. 2001). Cadmium, indium, palladium, and lead were historically used in some amalgam products. However, modern low-copper amalgams typically have a powder component composed of 69.4% silver, 3.6% copper, 26.2% tin, and 0.8% Zinc, they have a liquid component of 42% to 45% mercury by weight. Mercury amalgams are commonly used in dentistry because they are cheap, easy to use, durable, and regarded as safe. Although, for modern dentists who are exposed to mercury amalgam and vapor on a daily basis, no evidence of mercury poisoning have been demonstrated, some studies have shown that mercury from amalgams does affect dentists mildly. In several large-scale studies, dentists were asked to perform several cognitive and behavioral tests. Their tests results were compared to those of a normal population. The dentists lagged behind the normal population in many areas. In one study, 14% of dentists received worse scores in memory, coordination, motor speed, and concentration when exposed to an average personal air concentration time weighted average (TWA) [1]. A UK study that examines the health effects of mercury on 180 dentists, has found that the dentists who participated in the study had urinary mercury excretion levels four times of that of the control group on average. Dentists were significantly more likely than control subjects to have disorders of the kidney or memory disturbance [2].
Mercury from improperly disposed amalgam may be released into the air and sewage water. In various countries, amalgam removed from teeth is classified as hazardous waste. Environmental risks are mitigated provided that amalgams are disposed of properly. The American National Standards Institute with the American Dental Association issued a standard regarding the proper handling and disposal of amalgam waste [3].
Prior to this invention, amalgam waste, including unused amalgam would be temporarily stored in containers that could be tightly sealed to prevent the escape of mercury vapor. Normally, screw top high-density polyethylene containers are used for the collection of used amalgam capsules and other mercury contaminated wastes, such as contact and non-contact amalgam scraps, and used chair side traps. However, technicians in the dental facilities often leave the recycling containers open, causing mercury vapor to be released into the dental treatment room.
U.S. Pat. No. 4,698,018 by Nepault discloses a device for collecting dental amalgam, designed to prevent amalgam spills. In operation, a collection cup is allowed to rotate in only one direction, and disposes waste amalgam from the cup into a waste cylinder as it reaches the inverted position. The cup will automatically return to its original upright position after release of pressure. This device eliminates the need to close the waste container manually after each disposal, thus reduces the possibility of exposing the amalgam waste directly to open air. However, this device does not prevent mercury vapor from forming within the container during storage and does not have mechanisms to impede or prevent the mercury vapor from leaking into the treatment facility
Many materials can bind elemental mercury or mercury oxide. Scientist and engineers have tried to use these materials to remove mercury-containing waste from air or gas streams. For example, U.S. Pat. No. 6,719,828 by Lovell et al. disclosed a silicate based sorbent, used to absorb elemental mercury or oxidized mercury species such as mercuric chloride from a flue gas (e.g., SO2, NO, NO2, and HCl). This sorbent is prepared by first generating an exchange silicate substrate via ion exchange between a phyllosilicate substrate, such as vermiculite or montmorillonite, and a solution containing one or more groups of polyvalent metals such as tin, iron, titanium, manganese, zirconium and molybdenum, dissolved as salts. The sorbent is then made by adding sulfide ions to the exchanged silicate substrate in a controlled manner.
Flyash has also been used to remove mercury from streams of waste gases, U.S. Pat. No. 4,273,747 by Rasmussent and U.S. Pat. No. 4,273,747 by Knowles. Flyash is usually composed of common oxygenated inorganic compounds such as silica, aluminum, calcium, magnesium, iron, sodium, potassium, titanium, and sulfur. Aluminosilicate and calcium compounds often account for 80% of the mass. Because of the larger surface area per unit mass yield, flyash often acts as a sorbent for extremely fine particulates, fumes, or vapor phase species. U.S. Pat. No. 5,787,823 by Knowles teaches that largely anhydrous flyash has abilities to absorb gases or fumes such as vapor phase mercury from waste gas streams. Knowles also teaches that this ability of flyash is not dependent upon the flyash location with respect to the gas stream. Flyash, which is normally borne by the combustion gas stream, can be expected to absorb mercury at rates similar to flyash that is concentrated on the surface of a filter bag as a filter cake. This is because flyash, which is part of the filter cake, is very loosely bonded to other flyash particles, and forms a very porous mass. The exposed and available surface area for reactions of individual flyash particles in such a filter cake approximates the same as an individual flyash particle which is airborne. Particles ranging in size from about 1.0 μm to about 100 μm that are largely composed of aluminosilicate mineral phases, can also be used to capture mercury in a gas stream because the large surface area per particle due to porosity.
Modified active carbon, nano-materials and polymers have also been studied as mercury sorbent. US patent application by Olson et al. (US Pub. No. 2006/0048646) describes a sorbent made of active carbon modified by reaction with a halogen and/or halide promoter, which binds mercury. Others have experimented with nanocomposites that combines high-surface silica with photocatalytic properties of Titania for effective capture of elemental mercury vapor [4]. Another example of a mercury-binding sorbent is Vinylpyridine copolymer which showed a greater than 90% removal efficiency [5].